1. Field of the Invention
The present invention relates to improvement on oxygen sensors, which each comprise a planar sensor element capable of generating electromotive forces between electrodes or capable of varying an electric resistance between them depending upon the concentration of oxygen in exhaust gases, a metallic housing member for housing the sensor element, a ceramic powder placed in the metallic housing member and adapted for fixing the planar oxygen sensor element and forming a gas-sealed section, and ceramic supporters for compacting the ceramic powder under a given pressure.
2. Related Art Statement
As shown in FIG. 5a, a ceramic powder 30 such as talc is filled between a metallic cap 10 and a sensor element 12, the sensor element 12 is fixed by compacting the ceramic powder under pressure, and air is used as a reference oxygen atmosphere. In such an oxygen sensor, the air as the reference oxygen atmosphere is isolated, by means of the compacted ceramic powder 30, from a gas to be measured.
Ceramic supporters 28: 28a, 28b and 28c are used for forming the gas-sealed section by compacting the ceramic powder such as talc under pressure, and as shown in a perspective view of FIG. 5b by way of example, that portion of each of the ceramic supporters which contacts the ceramic powder 30 such as talc is flat.
However, in the oxygen sensor of this construction, when the sensor undergoes heating, a gap is formed between the lower end surface of the ceramic supporter 28a and the cap 10 and/or between the upper end surface of the ceramic supporter 28a and the talc 30a, due to difference in thermal expansion between the cap 10 and the ceramic supporter 28a. At that time, when vibrations are applied to the sensor, the ceramic supporter 28a vibrates to vibrate the sensor element 12. As a result, the sensor element is vibrated, and fixing forces and the gas-tightness between the sensor element and the talc are deteriorated.